A soldered connection may be created by assembling the parts to be joined with a solder preform, heating the assembly to a temperature where the solder preform melts, and then allowing the assembly to cool. This process is generally termed reflow soldering.
As in all soldering processes, it is important that the solder and the parts to be joined be free of oxidation that inhibits wetting of the parts by the molten solder and creates defects, such as voids and inclusions in the soldered joint. Typically a chemically active flux is used to remove and prevent oxidation of the molten solder. After soldering, the residual flux leaves a corrosive residue that should be removed to provide increased reliability of the assembly.
It is desirable to be able to perform soldering operations without the need for flux because of the possibility of residual corrosive contamination. This is particularly desirable for electronic assemblies, especially microelectronic assemblies. In microelectronic assemblies soldered joints may be made on or in close proximity to delicate structures such as bonding wires. Semiconductor chips may be soldered directly to substrates.
It is known that soldering in an oxygen-free atmosphere can eliminate the need for flux. One method for providing an oxygen-free atmosphere is to provide an inert or reducing gas around the parts to be joined. Another method is to perform the soldering operation under a high vacuum. The reflow soldering process is particularly suitable for use in a vacuum because it is not necessary to physically manipulate the parts during the soldering operation.
To carry out reflow soldering under a vacuum, the parts may be placed in a pressure vessel that includes a heating element to form a furnace. The pressure vessel furnace is then evacuated and the parts heated to form the soldered connection. U.S. Pat. No. 3, 982,887 to Kendziora et al. shows a pressure vessel furnace for flux-free soldering. The Kendziora furnace uses a series of belt and roller conveyors to move the workpieces into and through the furnace. This provides a satisfactory device for workpieces of substantial size, particularly where some mechanical arrangement holds the parts in position prior to the formation of the soldered connection, so that the vibration and jostling inherent in the conveyor mechanism does not displace the parts prior to soldering. In particular, there is a discontinuity in the conveyor system to permit gates to seal the furnace for evacuation. This discontinuity is likely to create a particulary large mechanical shock to the parts as they enter the furnace. This makes the Kendziora furnace unsuitable for processing microelectronic assemblies where a slight vibration or shock can displace the unconnected parts sufficiently to produce a defective assembly.